A major process which is now industrially used for production of hydrogen peroxide is a process using an anthraquinone compound as a reaction medium, which is generally referred to as the anthraquinone process. An anthraquinone compound is used by being dissolved in an appropriate organic solvent. For this purpose, organic solvents may be used either alone or as a mixture, and it is common to use a mixture of two types of solvents, i.e., a polar solvent and a nonpolar solvent. A solution prepared by dissolving an anthraquinone compound(s) in an organic solvent(s) is referred to as a working solution.
The anthraquinone process involves a reduction step where an anthraquinone compound(s) in the above working solution is hydrogenated in the presence of a catalyst to generate an anthrahydroquinone compound(s), followed by an oxidation step where the anthrahydroquinone compound(s) is oxidized with air or with an oxygen-containing gas and thereby converted back into the anthraquinone compound(s) simultaneously with the generation of hydrogen peroxide. Hydrogen peroxide generated in the working solution is usually extracted with water and separated from the working solution. The working solution from which hydrogen peroxide has been extracted is returned again to the reduction step, thereby forming a cycle process. This process allows production of hydrogen peroxide substantially from hydrogen and air, and is therefore a very efficient and effective process. Using this cycle process, hydrogen peroxide has already been produced on an industrial scale.
During repeated cycles of hydrogenation and oxidation, organic solvents in the working solution will also be altered and accumulated as by-products. These by-products are mainly composed of ketone forms generated upon dehydrogenation of higher alcohols which are polar solvents in the working solution. A working solution enriched with ketone forms reduces the desired content of water which can be incorporated thereinto. As a result, such by-products lead to deterioration of catalytic activity and may cause an obstacle to safe and stable operation. Moreover, these by-products also include oxidized and decomposed products of organic solvent components. More specifically, carboxylic acids, polyols, phenols and so on are detected. These by-products will alter the various physical and chemical properties of the working solution, including specific gravity, viscosity, surface tension, etc., and therefore will cause a reduction in the production efficiency of hydrogen peroxide through the steps of reduction, oxidation and extraction of the working solution.
Until now, some processes have been proposed to remove various by-products from a working solution which has been used for many years and to regenerate such a working solution. A known example is a process in which the reduced anthraquinone compounds are contacted with an aqueous alkaline solution to thereby remove by-products of the anthraquinone compounds, which do not contribute to the generation of hydrogen peroxide.
Likewise, a known example based on chemical treatment is a process in which a halide of aluminum, ammonium or the like is used in solid or aqueous solution form to treat a working solution at 100° C. to 170° C. and thereby regenerate by-products of anthraquinone compounds.
In addition, another known example is a process in which a working solution is treated with ozone and then extracted with an aqueous solution of an alkali metal hydroxide, and the working solution thus extracted and separated is contacted with activated alumina or activated magnesia.
On the other hand, a process is also known which comprises conducting the first stage of distillation to separate organic solvents from a working solution and then conducting the second stage of distillation to separate anthraquinone compounds and monoanthracene-based low-boiling substances, during which blockage caused by crystallization of distillates is prevented.
In addition, Patent Document 1 discloses a process for production of hydrogen peroxide, characterized by having a first distillation step of recovering organic solvents from a working solution by distillation performed at an atmospheric pressure or a lower pressure and a second distillation step of, following the first distillation step, recovering anthraquinone compounds by distillation performed at a still lower pressure at a temperature of 200° C. or higher for a residence time of 1 hour or longer, and characterized by treating the working solution prepared from all the distillates with a regeneration catalyst.
Patent Document 2 discloses a process in which organic solvent-derived low-boiling denatured substances are removed by azeotropic distillation with an aqueous alkaline solution.
In addition, Patent Document 3 discloses a process in which a working solution is distilled and the resulting organic solvents are treated by being contacted with water to remove organic solvent-derived denatured substances.